This appolication solicits support for the development and implementation of a randomized controlled clinical trial to study the effect of a low fat diet (20% of dietary calories) on breast cancer incidence. In this application as a clinical unit, we have a cohort of 10,543 women ages 45-70 who have participated in a mammagram breast cancer detection program and who are being followed up, whose annual risk of breast cancer is estimated to be about 1%. We know that of these 10,543 women in the age group 45-70: 1) 288 had a mother or sister with breast cancer, and two or more biopsies for benign disease, and/or first pregnancy over age 30; 2) 38 patients had both mother and sister with breast cancer; 3) 69 women not in categories 1 and 2 above had a high risk biopsy (1973-1983) of a) atypical hyperplasia, b) fibroadenoma, c) intraductal carcinoma, d) invasive breast cancer less than 5mm in size. In addition to these 395 high risk women, who are already identified and are being followed, we have available an additional group of 269 consultation subjects who meet the risk criteria for this study, providing us with a total of 664 subjects for study recruitment. For the feasibility study, we plan to enroll between 200 and 300 women from our at-risk subject pool of 664. We plan to institute and monitor a low fat (20%) diet with longitudinal follow up of both the special diet group as well as the customary control diet group. We have in place through our Breast Consultation Center (an outgrowth of the Breast Cancer Detection Demonstration (BCDD) Project) a standard procedure for baseline and annual physical examination and mammography. Of equal importance, we have the experience for ongoing annual long term follow up of our 10,543 enrolled screenees: 64 patients have been lost to follow up (0.6%); 303 are alive, but refuse to divulge further information (2.9%); 81 transferred to another BCDD in another geographic area of the country (0.76%); and 315 are dead (3.0%). We agree to work cooperatively with NCI staff in the implementation and conduct of the study, including design of the feasibility study, and if practicable, the longitudinal 5 year study.